Implanted medical devices, such as pacemakers, may deliver appropriately timed electrical pulses designed to cause the heart to contract or beat, i.e., to “capture” the heart. The stimulation pulses provided by implanted pacemakers usually have well-defined amplitude and pulse width characteristics, which can be adjusted to meet physiologic and device power conservation needs of a particular patient.
The amplitude and pulse width of the pacing pulses must be of an energy magnitude above a stimulation threshold in order to maintain capture of the heart. In order to prolong battery life, however, the energy magnitude of the pacing pulses should not be higher than the stimulation threshold by more than is needed for a reasonable safety margin.
The stimulation thresholds in the atrium and ventricle of a patient often fluctuate in the short term. For example, stimulation thresholds may decrease with exercise and may increase with various other activities, including sleep. Further, stimulation thresholds in the atrium and ventricle may gradually change in the long term. For instance, inflammation in the cardiac tissue around a tip of a pacing lead electrode drives the stimulation threshold up sharply during the first few weeks after implantation of the pacemaker, in turn requiring greater pacing pulse energy to maintain capture. Some of the inflammation reduces over the long-term, causing the stimulation threshold to decrease.
The energy magnitude of the pacing pulses may be determined after implantation and may be adjusted in accordance with the changing stimulation thresholds by performing stimulation threshold tests. Stimulation threshold tests include applying a test pulse and waiting for a cardiac sense in response to the test pulse. A cardiac sense indicates that the test pulse captured the heart and the energy magnitude of the test pulse is above the threshold. Alternatively, failure to detect a cardiac sense indicates that the test pulse did not capture the heart and the energy magnitude of the test pulse is below the threshold. Test pulses are applied until the stimulation threshold, i.e., the point at which a lesser energy pulse results in loss of capture, is found.